1. Field of the Invention
The present invention relates to a circuit for driving a stepping motor to return efficiently a needle of a meter used especially in a vehicle to a zero position when starting to drive.
2. Description of the Related Art
Meters mounted in a vehicle, such as a speedometer to indicate a vehicle drive speed or a tachometer to indicate an engine rotation speed or the like have recently used a stepping motor to meet the requirement of response speed or indicating accuracy. However, for indicating preciously with a stepping motor, synchronizing of timing, on which a stopper piece rotating together with a stepping motor abuts on a fixed stopper, and a specific step of excitation signals to drive the stepping motor is required.
Such synchronization is usually accomplished by sensing a timing on which said stopper piece abuts on the fixed stopper to monitor induced voltage in coils by a stepping motor rotation when a battery is turned ON or ignition is switched ON/OFF.
Operation of the return of a needle or a stopper piece to a zero position defined by a fixed stopper and synchronizing a timing to abut on the fixed stopper with a specific step of excitation signals is called a xe2x80x9creturn-to-zero processxe2x80x9d or simply xe2x80x9creturn-to-zeroxe2x80x9d herein.
The method of sensing induced voltage in the prior art, as mentioned above, has several drawbacks, such as requiring much time for return-to-zero process at battery turn on; needle continuously oscillating each time the ignition is turned ON/OFF; requiring much time for return-to-zero because of the necessity for keeping suitable rotating speed of a rotor to sense induced voltage securely; indication error caused by an inaccurate stepping motor; imprecise judging to abut on a stopper by means of unclear distinction of induced voltage level on a stepping motor with a rotor of a large numbers of magnetized poles and a large reduction gear ratio or complicated circuit required by induced voltage sensing coil and a sensing circuit.
The present invention has been accomplished to overcome the above drawbacks and an object of the present invention is to provide a drive circuit for a stepping motor to eliminate sensing function for induced voltage and to reduce any oscillatory motion of a needle on return-to-zero process and to reduce process time with a simple circuit structure.
In order to attain the above objects, a drive circuit for a stepping motor according to the present invention controls a position of a needle of a meter mounted in a vehicle by needle control signals and comprises an excitation signal generating means for generating excitation signals, wherein one electrical cycle is formed by a plurality of excitation steps supplied in drive coils to rotate a rotor with N-poles and S-poles, magnetized equally and alternately, of a stepping motor and a first excitation signal supply controlling means for controlling the excitation signal generating means to supply the excitation signals to the drive coils, wherein said needle is connected to the rotor through a speed change gear and rotates correspondingly to the rotor rotation, so that the excitation signals correspond to a needle rotating angle more than a maximum needle rotating angle toward a zero position to respond to the needle control signals generated at a timing when a vehicle battery turns ON.
According to the present invention, the excitation signals corresponding to rotating the needle with an angle more than the maximum needle rotating angle toward a zero position are supplied to the drive coils by means of controlling the excitation signal generating means with the first excitation signal supply controlling means at a timing when a car battery turns ON. When the excitation signals corresponding to an angle more than the maximum needle rotating angle are supplied to the drive coils, the needle is always set at the zero position.
Therefore, setting a zero position is accomplished without sensing induced voltage and then a time for returning the needle to the zero position is extremely shortened. Then, the circuit structure becomes simpler and unit size becomes smaller because sensing coils for induced voltage and sensing circuit are not required. Furthermore, in case of using an inaccurate stepping motor or a stepping motor with a rotor have a large number of magnetized poles and a large reduction gear ratio, return-to-zero can be done securely. Furthermore, when connecting a vehicle battery or replacing it, the present invention is very effective to set a zero position of a stepping motor.
In order to attain the objects, a circuit for driving a stepping motor according to the present invention is for controlling a needle position of a meter mounted in a vehicle by needle control signals and comprises an excitation signal generating means for generating excitation signals, wherein one electrical cycle is formed by a plurality of excitation steps supplied in drive coils to rotate a rotor with N-poles and S-poles, magnetized equally and alternately, of a stepping motor and a first excitation signal supply controlling means for controlling the excitation signal generating means to supply the excitation signals to the drive coils, wherein said needle is connected to the rotor through a speed change gear and rotates correspondingly to the rotor rotation, so that said excitation signals correspond to twice said one electrical cycle rotating toward a zero position to respond to the needle control signals generated at a timing when an ignition turns ON or OFF.
According to the present invention, the excitation signals corresponding to twice one electrical cycle rotating the needle toward a zero position are supplied to the drive coils by means of controlling the excitation signal generating means at a timing of turning ignition ON or OFF by the first excitation signal supply controlling means. When the excitation signals corresponding to twice one electrical cycle are supplied to the drive coils, the needle is almost usually set at the zero position.
Therefore, setting a zero position is accomplished without sensing induced voltage and viewing improves because a needle is not oscillating continuously each time the ignition is turned ON/OFF. Furthermore, the circuit structure becomes simpler and unit size becomes smaller because sensing coils for induced voltage and sensing circuit are not required.
In order to attain the objects, a drive circuit for a stepping motor according to the present invention has an excitation signal generating means, comprising a filtering means for filtering the needle control signals to move the needle faster to the zero position, and a SIN/COS table for generating SIN/COS data corresponding to the excitation signal of micro-steps based on filtering result by means of the filtering means and an output circuit for outputting excitation signals supplied into the drive coil as a required voltage value to rotate the rotor based on SIN/COS data by the SIN/COS table.
According to the present invention, because the excitation signal of micro-steps is supplied into the drive coil by the filtering means, the SIN/COS table and the output circuit in the excitation signal generating means, the oscillatory motion of the needle during return-to-zero is reduced. Because the vibrated motion of the needle becomes small, the needle oscillation may cease completely.
In order to attain the above objects, the drive circuit for a stepping motor according to the present invention, has a filtering means comprising a first filter for filtering the needle control signals to add or subtract predetermined value corresponding to change values of the needle control signals, a second filter for smoothing data change value to weighted-average filtering results by the first filter every predetermined time period and a third filter for weighted-averaging filtering results by the second filter every predetermined time period to prevent out-of-step of a stepping motor and sending the data to the SIN/COS table.
According to the present invention, because the needle control signals are filtered by two steps of the first filter and the second filter, the change values of the needle control signals are smoothed. Furthermore, out-of-step of a stepping motor is prevented by weighted-averaging two steps filtered needle control signals by the third filter. Thus, return-to-zero is done smoothly and securely.
In order to attain the above objects, the rotor is magnetized equally and alternately in N-pole and S-pole to pair of 5 poles, wherein the excitation signals is generated to be formed with equally-divided 32 excitation steps, based on a sine wave shape.
According to the present invention, the rotor is magnetized equally alternately in N-pole and S-pole to pair of 5 poles and the excitation signals are generated to be formed with equally-divided 32 excitation steps and then return-to-zero is performed smoothly as shown in FIGS. 6 and 7.
In order to attain the above objects, the drive circuit for a stepping motor according to the present invention comprises a second excitation signal supply controlling means for controlling the excitation signal generating means to supply to the drive coils the excitation signals for moving the needle in an amount based on a measured signal toward the zero position or counter to the zero position, in response to the needle control signals generated based on the measured signals supplied by a measurement unit in a vehicle.
According to the present invention, the second excitation signal supply controlling means controls the excitation signal generating means to supply to the drive coils the excitation signals for moving the needle in an amount based on a measured signal. The similar excitation signals used for return-to-zero can be also used for indicating in a meter, responding normal measured signals. Therefore, the same rotation controlling of the rotor is applied on return-to-zero and normal condition and then the processing can be simplified and a CPU load for rotation controlling of a rotor is enhanced.
In order to attain the above objects, the drive circuit for a stepping motor according to the present invention comprises the drive circuit for a stepping motor mentioned above, wherein an amount of the excitation signal corresponding to rotating the needle more than the maximum needle rotating angle is the amount to rotate the needle 360 degree.
According to the present invention, the excitation signal corresponding to rotating the needle 360 degree toward the zero position is supplied to the drive coils. Therefore, a needle of any meter is always set on the zero position securely since usually a needle does not rotate over 360 degree in maximum and the excitation signal corresponding to rotating the needle 360 degree is supplied into the drive coils.